refactor: MajorityElement

[alxkm]

Added null and empty input check to prevent unnecessary processing or NPEs.

Switched to interface type (Map) for better abstraction and flexibility.

Fixed JavaDoc to clarify behavior when input is null or empty.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.62%. Comparing base (ef93cc1) to head (6b9122d).

Files with missing lines	Patch %	Lines
...atastructures/hashmap/hashing/MajorityElement.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6380      +/-   ##
============================================
- Coverage     74.64%   74.62%   -0.02%     
  Complexity     5445     5445              
============================================
  Files           682      682              
  Lines         19000    19002       +2     
  Branches       3668     3668              
============================================
- Hits          14182    14181       -1     
- Misses         4260     4261       +1     
- Partials        558      560       +2     

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